In a common process for the manufacture of pulp for producing paper, logs are reduced to chips by chipping mechanisms, and the chips are cooked with chemicals at elevated pressures and temperature to remove lignin. The chipping mechanisms produce chips which vary considerably in size and shape. For the cooking process, which is known as digesting, it is desirable that the chips supplied have a uniform thickness in order to achieve optimum yield and quality; that is, to obtain a pulp which contains a low percentage of undigested and/or over-treated fibers. Under preferred conditions of digesting, the pulping chemicals or liquor penetrate into chips uniformly. If chips are provided which have too great a thickness, the liquor may not adequately penetrate the chips and the digester will produce chips with a core of under-digested fibers. If chips are provided which are too thin, the digester will produce chips that are overcooked and of low quality. To insure proper delignification of the chips in the production of pulp, the supply should not contain chips having an excessive thickness which will give rise to lack of adequate penetration during the digestion process, nor chips which are overly thin and may be over-treated during the digestion process.
Two types of apparatus has been provided heretofore for screening chips to separate the over-thick and under-thick chips from those within the desired thickness range. One type of screening device is a disk screen. A disk screen has a plurality of generally circular disks mounted on parallel, rotating shafts. The disks are mounted coaxially on each shaft and spaced from each other, and the disks interleave with the disks of adjacent shafts to form screening gaps between the disks of one shaft and the disks of adjacent shafts. Through proper disk spacing, the screen can be used to separate either under-size or over-size chips from a stream of chips supplied to the screen.
A second type of screening apparatus for wood chips or the like which has substantially higher industrial capacity than a disk screen is a bar screen. A bar screen has a screening deck or bed which extends substantially horizontally, thus providing a large screening area. Chips are distributed across a receiving end of the screening deck, which is formed by a series of parallel bars having a particular top shape. Relative oscillatory motion is effected between sets of bars for effecting screening and moving the chips in a forward direction.
Bar screens have also been found to be useful for separating refuse and trash as an important step in recycling such materials.
Known bar screens separate a flow of material into two streams, an accept stream and a reject stream. In many circumstances, the reject stream will be further processed. Further processing of the reject stream would be greatly aided by an ability to divide the rejected stream into oversized and grossly oversized materials.
In processing municipal waste and the like, the spacing of the screen bars may need to be adjusted from one lot of material to another. On conventional bar screens, bar spacing can require the change-out of a bar positioning and retention member.
Yet another problem associated with known bar screens is the difficulty of aligning the interleaved sets of bars so that the space between bars is even and does not vary between the front and back of the bar screen.
What is needed is an improved mechanism for clamping bar screens to bar retention members which allows their ready replacement and adjustment Further, a bar screen which separates the rejected material into oversized and grossly oversized is needed. Still further, an adjustment mechanism is needed which allows one person to adjust the spacing between the interleaved bars of the two bar racks of a bar screen.